[Gasification] Grates and stuff

[Peter Singfield]

At 05:22 PM 5/2/2007 -0400, Jeff Davis wrote:
>Toby wrote:
>>   There are so many off  topic rants that on topic discussion gets no
>> reply.  Are there other technical blogs with this topic that one can ask
>> a question and get a reply?  Please help me find one that is relevant to
>> gasification technology so I can quit this list.
>
>Dear Toby,
>
>I have noticed that Jim M and I (and others) have wrote a lot of
>gasification e-mails that never get a reply. The one exception was when
>you replied to my grate design. There seems to be a lack of interest in
>this subject. I wonder why.
>
>
>Jeff
>

OK guys -- have appended what I can reach in and grab with but 10 seconds
-- no more -- of going through my Gas list archives on hard drive.

Now imagine iof they ever had the entire gas archives mounted with a good
search engine --

Peter -- Belize

*************appended**********

From: Reedtb2 at cs.com
Date: Sun, 3 Sep 2000 20:26:01 EDT
Subject: Re: GAS-L: The inverted downdraft gasifier
To: gasification at crest.org

Dear Vern and Gasification:

SV           Superficial velocity
AF           Air Fuel ratio
Ideal AF =   1.5 air/1 biomass (Dry basis)


I am wrestling with these same questions and hope to bring out eventually a 
Volume II of "Survey of Biomass Gasificatin - 2000"  (or 2001 or 2010, 
whenever I get it all sorted out).  Meanwhile, I am writing subsections 
covering exactly what Vern has asked about.

I have said that the superficial velocity (throughput = gas produced/area, in 
m/s, m3/m2-sec or hour or Btu or whatever) is critical in determining the 
output and behavior of any gasifier.  Equally important is the air-fuel ratio 
for gasification.  The ideal air/fuel is 1.5 kg air/kg biomass (dry basis) 
and this produces the lowest tar and charcoal, but the lowest (5 MJ/nm3) 
energy content in the gas.  SV and AF vary together in ways that I am putting 
together now.

PRODUCER GASIFICATION:
At at high SV, temperatures in the flaming pyrolysis zone can exceed 1000C 
and this can gasify the charcoal as it is produced.  Any excess heat in the 
gas is immediately converted to gas by passing through whatever charcoal 
remains, and so can give a final char yield well bewlow 5%.  I describe this 
operating point as the "sweet spot" of the gasifier.  As one drops below this 
intensity you get more tars and char.  

PYROLYTIC GASIFICATION:
At the other end of the SV and AF spectrum, (low SV, <0.05 and low A/F, <0.5) 
we enter what I would like to call "pyrolytic gasification" in which we 
produce lots of charcoal (10-25%) and tar (0.1-1%) and a rich gas (20 
MJ/nm3?).  The charcoal is valuable and the tar can be burned if you are only 
interested in heat applications, but for powering engines etc. the tar is a 
great nuisance.  

Since most people don't characterize their gasifiers in these terms, it is 
necessary to do some calculations to know where you are in this SV-AF 
spectrum.  

If this is less than crystal clear, my apologies.  My understanding increases 
daily as I work with the thermodynamics, kinetics and the actual gasifiers we 
are building and testing.  

Yours truly,                        TOM REED                        BEF/CPC

In a message dated 8/30/00 9:17:31 PM Mountain Daylight Time, 
VHarris001 at aol.com writes:

<< n a message dated 08/25/2000 9:31:07 AM Eastern Daylight Time, 
 Reedtb2 at cs.com writes:
 
 >                                     ~~~~~
 >  In the conventional downdraft gasifier air is drawn by a blower or engine 
 >  from nozzles (Imbert starting 1938) or from the top of a cylindrical tube 
 >  (stratified downdraft downdraft, starting at SERI, 1980).  It first meets 
 >  unburned biomass and burns it in the process I call "flaming pyrolysis".  
 >  (Flaming combustion is like the flame of the match; flaming pyrolysis is 
 >  similar, but has limited air.)   This produces a gas containing 
 considerable 
 > 
 >  CO, H2 and CH4, but also CO2 and H2O at temperatures of 1000-1500C and 
 >  produces from 5-25% charcoal, depending on the superficial velocity of 
the 
 >  gases.  
 >  
 >  These HOT gases pass over the resulting charcoal and are further reduced 
 by 
 >  the  charcoal to producer gas.  If the superficial velocity is about 0.4 
 m/s 
 > 
 >  and the gas has a heating value of 5 MJ/nm3, the output will be 2 MJ/m2-s 
 or 
 > 
 >  2,000 kWth/m2, or 0.2 kW/cm2, an amazing throughput.  
 >                                     
 
 Hi Tom (Reed),
 
 I continue to struggle with the definition of "gasification."  I note that 
 you again discuss pyrolysis gases, which are further reduced by passing 
 through a bed of hot charcoal, resulting in "producer gas."
 
 I was wondering if you would take the time to compare and contrast 
"pyrolysis 
 gases" and how they are produced versus "producer gases" and how they are 
 produced.
 
 Is a bed of charcoal, or other carbon bed, *required* in order to generate 
 "producer gas" in a gasifier?  If so, does that imply that energy sources 
 that do not produce a bed of charcoal when "gasified" (or contain little 
 moisture available for the water-gas reaction) are not actually being 
 "gasified?"  That is to say, if there is no charcoal present for reduction 
to 
 occur, is the resulting gas still considered "producer gas?" 
 
 If the off-gas isn't considered producer gas, then isn't there actually a 
 considerable degree of reduction and/or water gas reaction occuring (highly 
 endothermic, correct)?  On the other hand, if the off-gas is considered 
 producer gas, then isn't the charcoal bed somewhat superfluous to 
 gasification?
 
 Finally, your continued research into the subject indicates that superficial 
 velocity is of considerable importance in producing tar-free producer gas.  
 However, if the material being gasified does not produce a bed of charcoal, 
 will an appropriate superficial velocity still result in crack tars and thus 
 a low tar content pyrolysis gas?
 
 Thanks for your continued contributions to gasification!
 
 Best,
 
 Vernon Harris >>
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